Nuanced effects of organic matter on native phytoplankton and non-native Ceratium: A case study re-analyzing field data using non-linear thresholds

Anthropogenic pressures such as extreme climate events and biological invasions have led to declines in water quality and biodiversity. Using a 5-year monitoring dataset, from 2014 to 2018, in an artificial lake designated for drinking water supply, this case study aims to identify ecological thresholds of organic matter that lead to fundamental changes in phytoplankton biodiversity. We hypothesize that 1) native taxa would predominantly decline in frequency and abundance, while abundance of the invasive Ceratium would increase with higher organic matter concentration; and 2) significant negative impacts on native total abundance and richness (taxonomic and functional) would occur above the identified threshold. Regarding population-level changes, all sensitive taxa were native, whereas the invasive Ceratium showed tolerance, possibly through heterotrophic carbon assimilation. Overall, native taxa decline at 4.9 mgL^-1 whereas Ceratium significantly increases at 5.6 mgL^-1 along the organic matter gradient. At the community-level, native phytoplankton abundance, as well as taxonomic and functional richness, declined significantly above the identified community threshold of 5.25 mg/L. We call for a stronger understanding of the impacts of organic matter and biological invasions across different reservoirs, and recommend incorporating biomass data to improve the broader applicability of ecological thresholds.